Optical fiber Bragg grating or FBG is a short segment of optical fiber in which its refractive index in the core or cladding or both varies periodically along the fiber. When a broadband light is launched into an FBG, only the light at the resonance wavelength, which is also known as the Bragg wavelength, is reflected. The Bragg wavelength is given by:λB=2neffΛ
where neff is the effective index of refraction of the fiber and Λ is the grating period. It is clear that any change in neff or Λ will shift the Bragg wavelength. The most common parameters that may shift the Bragg wavelength may be temperature, strain and pressure. Therefore FBGs may be utilized as a sensor to measure these physical quantities. Besides these quantities that may directly change the Bragg wavelength, some other physical, chemical or biological quantities could also be measured by coating the surface of an FBG with a special material. The dimensions or temperature of the coating may change when the coating is exposed to a quantity. This change will then shift the Bragg wavelength of the FBG. For example, when an FBG is coated with a piezoelectric or magnetostrictive material, the coated FBG may then be utilized as a sensor to measure electric or magnetic fields. If an FBG is coated with palladium, the FBG may be utilized to measure hydrogen concentration. There may be many other examples to utilize an FBG for sensing of a variety of physical, chemical or even biological parameters.
It is also often desirable to multiplex a number of FBGs in a single optical fiber for quasi-distributed sensing. Typical methods for FBG multiplexing include the wavelength division multiplexing or WDM, the time division multiplexing or TDM, the optical frequency-domain reflectometry or OFDR, and various combinations of some of these methods.